The present invention relates to fiber optic cables, and, more particularly, to fiber optic cables including sets of optical fibers.
Conventional fiber optic cables include optical fibers that conduct light which is used to transmit voice, video, and data information. Where the fiber optic cable is subjected to forces, the optical fibers may be stressed and attenuation of the transmitted light may occur. It is therefore important for fiber optic cables to be constructed in a robust manner whereby stress-induced attenuation can be avoided or minimized. In addition, although it is generally desirable for a fiber optic cable to have a high optical fiber count, it is also desirable for the cable to be as compact as possible, thereby maximizing optical fiber density.
High fiber count cables can be classified into three general design categories, namely: single tube, stranded tube, and slotted core. Each category may include optical fiber ribbons and/or bundled optical fibers. The physical characteristics and/or optical performance of high fiber count cable designs can include, for example: general properties such as packing density, cable diameter, weight and flexibility; cable performance attributes such as environmental performance, mechanical performance, and polarization mode dispersion attributes; and field characteristics such as installation methods, cable stripping, and mid-span access.
Known cable designs that include optical fiber ribbons, and are classifiable into one of the three general categories, can define a backdrop for the present invention. For example, U.S. Pat. No. 5,608,832 which is incorporated by reference herein, includes a central member. More specifically, the design includes stacks of optical fiber ribbons formed by three optical fiber ribbons disposed in respective three-sided chamber elements of approximately a U-shaped cross section. The chamber elements are stranded around the central member which includes a tensile element and an extruded plastic layer. U.S. Pat. No. 5,249,249 and U.S. Pat. No. 5,293,443 which are respectively incorporated by reference herein, also disclose designs employing central members. The respective disclosures describe a compartment holding at least two side-by-side stacks of optical fiber ribbons.
U.S. Pat. No. 5,177,809 which is incorporated by reference herein, includes a slotted rod. Disclosed therein is an optical cable having a plurality of light waveguides in a group of bands that are arranged in longitudinally extending chambers of a slotted rod. Each of the chambers in the slotted rod can have an increasing width as the radial distance from the center of the slotted rod increases. The bands can be arranged in sub-stacks having increasing widths corresponding to the increased width of the chamber. In another embodiment, each of the bands in the stack has an increasing width in the radial direction to fill the chamber. Alternatively, each of the chambers has a rectangular cross section, with the cross section increasing in a step-like manner due to steps formed in partitions separating the chambers. The bands that are arranged in the chambers are arranged in sub-stacks to fill each portion of the chamber.
The background of the present invention can include single tube cable designs having optical fiber ribbons. For example, U.S. Pat. No. 5,369,720 which is incorporated by reference herein, discloses a stack of optical ribbons secured within a metal tube by an adhesive. The adhesive has a peel strength sufficiently low to permit separation of individual optical ribbons from the stack. One embodiment includes a stack of optical ribbons having a number of ribbons arranged generally parallel to each other, and a further pair of ribbons arranged perpendicular to the generally parallel ribbons and in abutment with edges thereof. In addition, U.S. Pat. No. 5,878,180 discloses a single tube cable including a number of superimposed and adjacent stacks of optical fiber ribbons. The stacks of optical fiber ribbons are arranged over and/or adjacent to each other and in parallel. Another single tube variation, is disclosed in EP-A2-0495241 wherein optical fiber ribbons are tightly received in a zigzagged waterblocking tape. The ribbons are apparently pressed into slots in the zigzagged waterblocking tape. The zigzagged waterblocking tape disadvantageously consumes valuable space inside the tube, increases production costs, requires specialized manufacturing procedures, restricts relative movement of the ribbons during cable bending, increases friction between cable components, and/or adds size and stiffness to the cable.
In addition to attaining a desired fiber count, fiber optic cables should be able to withstand longitudinal compression and tension, and they typically include strength members for these purposes. However, the strength members may disadvantageously affect cable bending performance during installation, and may hinder optical fiber access. A fiber optic cable having strength members located in a single plane generally will experience a preferential bending action favoring bending of the cable out of the plane defined by the strength members. On the other hand, a fiber optic cable having strength members at spaced locations encircling the center of the cable will not have a preferential bend, but the strength members typically include a helical lay so that the cable can be bent. Even taking into account the helical lay of the strength members, when bent in generally any axis, cables of the non-preferential bend type may be very stiff, a characteristic which may be highly undesirable depending upon installation requirements. Thus a cable of the preferential bend type will typically experience ease of cable bending in a preferred plane, and, as there are less strength members to deal with, may present a less time consuming optical fiber access procedure. A cable designer may therefore balance the need to have sufficient cable components for resisting crush, compression, and tension loads, against the size and stiffness contributions of the cable components that may render the cable difficult to install in a cable passageway.
The present inventions relate to a fiber optic cable having a tube assembly therein. The tube assembly includes an optical fiber group in a tube. The optical fiber group comprises a medial optical fiber subgroup and lateral optical fiber subgroups adjacent thereto. The subgroups define a step-like profile for maximizing optical fiber packing density of the tube assembly and/or defining a high fiber count cable. In exemplary mono-tube embodiments, a diagonal free space is defined as the tube inner diameter minus the diagonal length of the cross-section of the profile of the optical fiber ribbon stack, the diagonal free space being about 2 mm to about 5 mm. In a multi-tube embodiment, diagonal free space can be about 0.5 mm to about 2 mm. In other embodiments, corner fibers can have a delta optical attenuation of less than about 0.05 dB/Km for a wavelength of @1550 nm over a 100 meter length 40xe2x80x3 to 70xe2x80x3 drum at room temperature.